LC Transition Research Articles

Temperature Dependence of Molecular Orientation on the Surfaces of Semifluorinated Polymer Thin Films

Near-edge X-ray absorption fine structure is used to investigate the temperature dependence of molecular orientation of semifluorinated liquid crystalline (SF-LC) mesogens, which are attached to the modified isoprene backbone of (1) a poly(1,2-isoprene) homopolymer and (2) a diblock copolymer consisting of polystyrene and poly(1,2-isoprene) blocks. These experiments reveal the existence of two temperature regions in which the surface orientation of the SF-LC mesogens changes abruptly, but even 30 K above the highest such temperature region the surface orientation does not become isotropic. The lower temperature surface transition for both homopolymer and block copolymer occurs close to the temperature of the bulk homopolymer smectic-B to smectic-A transition and well above the bulk smectic-B to smectic-A transition in the block copolymer. It seems to be controlled exclusively by the ordering phenomena originating from the surface. In contrast, the change in the surface organization of the SF-LC mesogens at higher temperatures can be associated with bulk LC transition from the smectic-A to the isotropic phase.

Studying Polymer-Dispersed Liquid-Crystal Formation by FTIR Spectroscopy. 2. Phase Separation and Ordering

Polymerization-induced phase separation to form polymer-dispersed liquid crystals (PDLCs) is a complex process involving simultaneous curing and phase separation to form nematic microdomains. Real-time FTIR spectroscopy has been used to simultaneously observe and quantify the curing process using a model systema fast photocurable matrix (NOA65) and a liquid crystal (E7). While the role of FTIR spectroscopy in monitoring chemical changes is well recognized and reinforced for PDLCs, it is demonstrated in this paper that it is also a powerful tool to monitor physical changes (phase separation and nematic ordering). Phase separation was detected during the curing process by a scattering-induced change in the absorbance spectrum of the sample. Nematic ordering could be observed and quantified based on a change in a characteristic band of the liquid crystal. Moreover, the phase separation and the onset of nematic ordering are temporally resolved. The conversion at phase separation decreases strongly with an increase in liquid-crystal content, while the conversion required for phase separation increases with increasing temperature. Isotropic droplets are formed followed by nematic ordering in the domains at lower E7 concentrations, while the processes are simultaneous for higher concentrations. Temperatures close to and above the LC transition temperature also lead to isotropic domains, which form nematic domains upon cooling. The fraction of liquid crystal present as nematic droplets and total fraction of nematic domains in the PDLC are quantified based on changes in the vibrational spectrum of E7. On the basis of mass balances applied to the closed process, the phase diagram of the system could be determined as a function of curing temperature. Solubility limits obtained by this method agree well with results obtained by other researchers. The concept of a spectroscopic composite plot that completely describes the formation process in a PDLC is proposed.

X-Ray Scattering and Thermal Characterization of α,β-Dimethyl Stilbene Polycarbonates

Abstract Structural and thermal characterization is reported for a series of recently synthesized1 liquid crystalline polycarbonates based on di-methyl substituted stilbene mesogen and methylene-containing flexible spacer of length n. In this work, we focus on the α, β-dimethyl stilbene (DMS-n) polycarbonates and on the comparison between them and α-methyl stilbene (HMS-n) polycarbonates2–4, especially those with heptane spacer. Wide angle X-ray scattering patterns show that DMS polycarbonates are able to form a nematic liquid crystalline phase. However, the liquid crystalline phase is generally not stable with respect to the crystalline phase. In the DMS polycarbonates, no distinct isotropic-to-nematic (i → lc) transition is seen either by polarizing optical microscopy or by differential scanning calorimetry (DSC). We suggest that the steric effect of the second lateral substituent results in a less stable, virtual mesophase. For DMS-n, an odd-even effect is observed in the crystallization and melting te...

Nonionic Surfactant Reverse Micelles of C12E4 in Dodecane: Temperature Dependence of Size and Shape

We have determined the boundaries of the single-phase water-in-oil microemulsion region of a ternary system (tetraethylene glycol monododecyl ether−dodecane−water) in the oil-rich corner of the phase diagram, as a function of water content and temperature. We have investigated the structures of reverse micelles at temperatures between the lamellar (LC) phase transition and the two-phase separation. Characterization studies have been carried out by dynamic quasi-elastic light scattering and small-angle X-ray scattering as well as by viscosity measurements. It was found that a consistent interpretation of the experimental results requires the complementarity of the above techniques. At a constant water-to-surfactant molar ratio of 13.7, the emerging picture of the isotropic phase is that near the two-phase boundary, micelles behave as slightly solvated spheres of 52-Å radius. With decreasing temperature and as the LC transition is approached, prolate aggregates, displaying an asymmetry of 6 ± 2 are formed and can be considered to be a progressive structural change on the way to lamellae. The physical properties of the system reported herein may have implications for the investigation of proteins in membrane-mimetic media.

Nematic transition in the melts of polymer chains containing rigid and flexible fragments

The nematic transition in the melt of polymer chains with alternating rigid and flexible segments is considered. The Mayer-Saupe midfield method is applied to such a system. The free energy of the system was calculated and the link between temperature and the averaged parameter of order in the state of thermodynamic equilibrium was found; the temperature of the equilibrium phase transition was determined. The effect of the parameters of the system on the LC transition was studied. Lengthening of the rigid portions and enhancement of the orientational interaction between them led to an expansion of the temperature region of the existence of the nematic phase. The degree of orderliness of the rigid portions close to the point of the phase transition, as a rule, well exceeded that of the flexible spacers.

Theory of elastic light scattering in solutions of semiflexible macromolecules in the region of the liquid-crystalline transition

A study has been made of the theory of elastic light scattering from an isotropic solution of semiflexible persistent macromolecules when approaching the LC transition point. The differential cross-section of scattering per unit volume has been calculated for scattering at an arbitrary angle and with an arbitrary value of the scattering wave vector q. In addition, the degree of depolarization of scattered light was calculated. The analytical results for small q are discussed as well as the accurate results obtained with the aid of a computer. The calculated data are compared with experimental results.